Rotor core

ABSTRACT

A rotor core which includes: a first hole portion group; and a second hole portion group. Each hole portion of the second hole portion group is arranged to intersect with a virtual extension line of a rib formed between the adjacent hole portions of the first hole portion group, and includes: a first and second end portions; an outer radial side apex portion; and an outer peripheral wall. The outer radial side apex portion is located at an intersection point between a virtual line which is orthogonal to a virtual line connecting the center of the rotor core and the first end portion and passes through the first end portion and a virtual line which is orthogonal to a virtual line connecting the center of the rotor core and the second end portion and passes through the second end portion or is located radially outward of the intersection point.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims the benefit of priority of JapanesePatent Application No. 2018-192066, filed on Oct. 10, 2018, the contentof which is incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to a rotor core which constitutes a rotorof a motor.

BACKGROUND ART

WO 2011/077522 discloses a rotor core which includes a rotor shaft holeinto which a rotor shaft is tightened, a first hole portion groupprovided on an outer side of the rotor shaft hole in a radial directionand having a plurality of hole portions arranged in a circumferentialdirection, a shaft holding portion provided between the rotor shaft holeand the first hole portion group in the radial direction, a second holeportion group provided on an outer side of the first hole portion groupin the radial direction and having a plurality of hole portions arrangedin the circumferential direction, a first annular portion providedbetween the first hole portion group and the second hole portion groupin the radial direction, and an electromagnetic portion provided on anouter side of the second hole portion group in the radial direction andhaving a plurality of magnet insertion holes into which magnets arerespectively inserted.

In this type of rotor core, the first hole portion group, the secondhole portion group, and the first annular portion function as regionsfor absorbing the centrifugal force of the rotor core. Further, in therotor core described in WO 2011/077522, each hole portion of the secondhole portion group is disposed so as to intersect with a virtualextension line of a rib formed between adjacent hole portions of thefirst hole portion group. Therefore, the centrifugal force can beabsorbed by the hole portions of the second hole portion group, and thusthe transfer of the centrifugal force to the shaft holding portionthrough the rib can be reduced.

However, in the rotor core disclosed in WO/2011/077522, when thecentrifugal force acts on each hole portion of the second hole portiongroup, a large bending stress is generated in the region around acircumferential end portion on an outer circumferential side of the holeportion of the second hole portion group, and thus stress concentrationoccurs.

SUMMARY

The invention provides a rotor core capable of alleviating stressconcentration by suppressing a bending stress generated in a holeportion by a centrifugal force.

According to an aspect of the invention, there is provided a rotor coreincluding: a rotor shaft hole into which a rotor shaft is tightened; afirst hole portion group provided on an outer side of the rotor shafthole in a radial direction and having a plurality of hole portionsarranged in a circumferential direction; a shaft holding portionprovided between the rotor shaft hole and the first hole portion groupin the radial direction; a second hole portion group provided on anouter side of the first hole portion group in the radial direction andhaving a plurality of hole portions arranged in the circumferentialdirection; a first annular portion provided between the first holeportion group and the second hole portion group in the radial direction;and an electromagnetic portion provided on an outer side of the secondhole portion group in the radial direction and having a plurality ofmagnet insertion holes into which magnets are respectively inserted,wherein: each hole portion of the second hole portion group is arrangedto intersect with a virtual extension line of a rib formed between theadjacent hole portions of the first hole portion group; each holeportion of the second hole portion group includes: a first end portionand a second end portion forming both circumferential end portions; anouter radial side apex portion having a radial distance from a center ofthe rotor core longer than that of the first end portion and the secondend portion and forming a radially outer apex portion; and an outerperipheral wall having a first outer peripheral wall extending from thefirst end portion to the outer radial side apex portion and a secondouter peripheral wall extending from the second end portion to the outerradial side apex portion; and the outer radial side apex portion islocated at an intersection point between a virtual line which isorthogonal to a virtual line connecting the center of the rotor core andthe first end portion and passes through the first end portion and avirtual line which is orthogonal to a virtual line connecting the centerof the rotor core and the second end portion and passes through thesecond end portion or is located radially outward of the intersectionpoint.

Effects

According to the invention, the stress concentration can be alleviatedby suppressing the bending stress generated in the hole portion due tothe centrifugal force.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a front view of a rotor core according to a first embodimentof the invention;

FIG. 2 is a partially enlarged view of FIG. 1;

FIG. 3A is an enlarged view of a hole portion of a first hole portiongroup;

FIG. 3B is a view illustrating a force acting when an outer diameterside apex portion of the first hole portion group is located on an innerside than an intersection point X;

FIG. 3C is a view illustrating a force acting when the outer diameterside apex portion of the first hole portion group is located at theintersection point X;

FIG. 3D is a view illustrating a force acting when the outer diameterside apex portion of the first hole portion group is located on an outerside than the intersection point X;

FIG. 4A is an enlarged view of a hole portion of a second hole portiongroup;

FIG. 4B is a view illustrating a force acting on the hole portion of thesecond hole portion group;

FIG. 5 is an enlarged view of a hole portion of a third hole portiongroup;

FIG. 6 is a partially enlarged view of the rotor core according to amodification example of the first embodiment; and

FIG. 7 is a front view of the rotor core according to a secondembodiment of the invention.

DESCRIPTION OF EMBODIMENTS

Hereinafter, embodiments of the invention will be described based on theaccompanying drawings. The drawings are viewed in the direction of thereference signs.

First Embodiment

First, a rotor core according to a first embodiment of the inventionwill be described with reference to FIGS. 1 to 6.

Rotor Core

A rotor core 1 is configured by laminating a plurality ofelectromagnetic steel plates in an axial direction of a rotor shaft 2and constitutes a rotor of a motor together with the rotor shaft 2 and aplurality of magnets 3 assembled to the rotor core 1.

As illustrated in FIG. 1, the rotor core 1 has an annular shape in whicha rotor shaft hole 4 into which the rotor shaft 2 is tightened bypress-fitting is provided at a center CL. The rotor core 1 includes afirst hole potion group 6 having a plurality of hole portions 5 providedon the outer side of the rotor shaft 4 in a radial direction andarranged in a circumferential direction and a shaft holding portion 7provided between the rotor shaft hole 4 and the first hole portion group6 in the radial direction. Further, the rotor core 1 includes a secondhole potion group 9 having a plurality of hole portions 8 provided onthe outer side of the first hole portion group 6 in the radial directionand arranged in the circumferential direction and a first annularportion 10 provided between the first hole portion group 6 and thesecond hole portion group 9 in the radial direction. Further, the rotorcore 1 includes a third hole potion group 12 having a plurality of holeportions 11 provided on the outer side of the second hole portion group9 in the radial direction and arranged in the circumferential directionand a second annular portion 13 provided between the second hole portiongroup 9 and the third hole portion group 12 in the radial direction. Inaddition, the rotor core 1 includes an electromagnetic portion 15provided on an outer side of the third hole portion group 12 in theradial direction and having a plurality of magnet insertion holes 14into which the magnets 3 are respectively inserted.

The magnet 3 is, for example, a permanent magnet such as a neodymiummagnet. In the embodiment, one magnet pole portion 20 is configured bythree magnets 3 arranged in three magnet insertion holes 14 arranged ina substantially arc shape which protrudes inward in the radial directionof the rotor core 1.

The first hole portion group 6, the second hole portion group 9, and thethird hole portion group 12, and the first annular portion 10 and thesecond annular portion 13 formed by those hole portion groups 6, 9, and12 function as regions for absorbing the centrifugal force due torotation of the rotor and the tightening load of the rotor shaft 2.

Arrangement of Hole Portions

As illustrated in FIG. 2, a rib 16 is formed between adjacent holeportions 5 of the first hole portion group 6. Each hole portion 8 of thesecond hole portion group 9 is arranged to intersect with a virtual lineL1, which is a virtual extension line of the rib 16 passing through thecenter CL of the rotor core 1 and a circumferential center position ofthe rib 16. That is, the hole portions 5 of the first hole portion group6 and the hole portions 8 of the second hole portion group 9 arealternately arranged in the circumferential direction. As a result, thecentrifugal force can be absorbed by the hole portions 8 of the secondhole portion group 9 and the transfer of the centrifugal force to therib 16 can be suppressed.

Each hole portion 8 of the second hole portion group 9 of the embodimentis disposed so that the circumferential center position is on thevirtual line L1. Furthermore, each hole portion 8 of the second holeportion group 9 has a circumferential length longer than that of the rib16 and circumferentially overlaps both of the adjacent hole portions 5with the rib 16 interposed therebetween.

Further, in the embodiment, the virtual line L1 is a line passingthrough the center CL of the rotor core 1 and the circumferential centerposition of the rib 16 and a line passing through the center CL of therotor core 1 and the circumferential center portion of each magnet poleportion 20. That is, the virtual line L1 coincides with a d axis whichis a central axis of the magnet pole portion 20.

A rib 17 is formed between adjacent hole portions 8 of the second holeportion group 9. Each hole portion 11 of the third hole portion group 12is arranged to intersect with a virtual line L2, which is a virtualextension line of the rib 17 passing through the center CL of the rotorcore 1 and the circumferential center position of the rib 17. That is,the hole portions 8 of the second hole portion group 9 and the holeportions 11 of the third hole portion group 12 are alternately arrangedin the circumferential direction. As a result, the centrifugal force canbe absorbed by the hole portions 11 of the third hole portion group 12and the transfer of the centrifugal force to the rib 17 can besuppressed.

Each hole portion 11 of the third hole portion group 12 of theembodiment is disposed so that the circumferential center position is onthe virtual line L2. Furthermore, each hole portion 11 of the third holeportion group 12 of the rotor core 1 has a circumferential length longerthan that of the rib 17 and circumferentially overlaps both of theadjacent hole portions 8 with the rib 17 interposed therebetween.

Further, in the embodiment, the virtual line L2 is a line passingthrough the center CL of the rotor core 1 and the circumferential centerposition of the rib 17 and a line passing through the center CL of therotor core 1 and one circumferential end portion or the othercircumferential end portion of each magnet pole portion 20. That is, thevirtual line L2 coincides with a q axis separated by 90 degrees inelectrical angle with respect to the d axis.

The plurality of hole portions 5 of the first hole portion group 6, theplurality of hole portions 8 of the second hole portion group 9, and theplurality of hole portions 11 of the third hole portion group 12 arearranged at equal intervals in the circumferential direction. Therefore,respective hole portion groups 6, 9, and 12 can receive the centrifugalforce equally over the whole circumferential direction.

Shape of Hole Portion

As illustrated in FIG. 3A, each hole portion 5 of the first hole portiongroup 6 has a substantially triangular shape convex outward in theradial direction. The hole portion 5 of the first hole portion group 6has a first end portion 5 a and a second end portion 5 b which form bothcircumferential end portions and an outer radial side apex portion 5 cwhich has a radial distance from the center CL of the rotor core 1longer than that of the first end portion 5 a and the second end portion5 b and forms an apex portion on the radial outer side. Furthermore, thehole portion 5 of the first hole portion group 6 includes an outerperipheral wall 5 e which has a first outer peripheral wall 5 fextending substantially linearly from the first end portion 5 a to theouter radial side apex portion 5 c and a second outer peripheral wall 5g extending substantially linearly from the second end portion 5 b tothe outer radial side apex portion 5 c. Further, the hole portion 5 ofthe first hole portion group 6 includes an inner peripheral wall 5 hwhich is substantially orthogonal to the virtual line L2 and extendssubstantially linearly from the first end portion 5 a to the second endportion 5 b.

Therefore, each hole portion 5 of the first hole portion group 6 canreduce the centrifugal force transmitted inward in the radial directionof the rotor core 1 by deformation of the outer radial side apex portion5 c with respect to the centrifugal force. Therefore, it is possible tosuppress the widening of the rotor shaft hole 4 due to the centrifugalforce and to suppress the reduction of the interference due to thewidening of the rotor shaft hole 4.

Furthermore, since, in the hole portion 5 of the first hole portiongroup 6, the inner peripheral wall 5 h has a substantially straight lineshape substantially orthogonal to the virtual line L2, a force acting onthe inner peripheral wall 5 h when the centrifugal force acts on theouter radial side apex portion 5 c of the hole portion 5 hassubstantially no radial component at the circumferentially centralportion of the inner peripheral wall 5 h. Therefore, since thedeformation of the shaft holding portion 7 can be reduced, it ispossible to suppress the widening of the rotor shaft hole 4 due to thecentrifugal force and to suppress the reduction of the interference dueto the widening of the rotor shaft hole 4.

The hole portion 5 of the first hole portion group 6 has an outer radialside apex portion 5 c located on the virtual line L2 and has asymmetrical shape with respect to the virtual line L2.

The outer radial side apex portion 5 c of the hole portion 5 of thefirst hole portion group 6 is located at an intersection point X betweena virtual line L4 which is orthogonal to a virtual line L3 connectingthe center CL of the rotor core 1 and the first end portion 5 a andpasses through the first end portion 5 a and a virtual line L6 which isorthogonal to a virtual line L5 connecting the center CL of the rotorcore 1 and the second end portion 5 b and passes through the second endportion 5 b, or is located radially outward of the intersection point X.In the embodiment, the outer radial side apex portion 5 c of the holeportion 5 of the first hole portion group 6 is located radially outwardof the intersection point X.

As illustrated in FIG. 3B, when the outer radial side apex portion 5 cis positioned radially inward of the intersection point X andcentrifugal force F acts on the outer radial side apex portion 5 c, abending stress Se in the radially inward direction is generated in theouter peripheral wall 5 e in addition to a tension Te. Therefore, in theregion around the first end portion 5 a and the second end portion 5 bof the outer peripheral wall 5 e, the bending moment becomes large andthe bending stress is concentrated.

On the other hand, as illustrated in FIG. 3C, since, when the outerradial side apex portion 5 c is located at the intersection point X, thefirst outer peripheral wall 5 f is along the virtual line L4 and thesecond outer peripheral wall 5 g is along the virtual line L6, even whenthe centrifugal force F acts on the outer radial side apex portion 5 c,almost no bending stress occurs in the outer peripheral wall 5 e.Therefore, stress concentration in the region around the first endportion 5 a and the second end portion 5 b of the outer peripheral wall5 e can be alleviated.

Also, as illustrated in FIG. 3D, when the outer radial side apex portion5 c is positioned radially outward of the intersection point X, gaps areformed in a portion between the outer peripheral wall 5 e and thevirtual line L4 from the first end portion 5 a to the intersection pointX and a portion between the outer peripheral wall 5 e and the virtualline L6 from the second end portion 5 b to the intersection point X.Therefore, when the centrifugal force F acts on the outer radial sideapex portion 5 c, in addition to the tension being generated in theouter peripheral wall 5 e, the tension Th is generated in the innerperipheral wall 5 h. Thus, the stress generated in the hole portion 5 bythe centrifugal force F acting on the outer radial side apex portion 5 cis dispersed to the outer peripheral wall 5 e and the inner peripheralwall 5 h. As a result, the bending stress generated in the outerperipheral wall 5 e can be reduced and the stress concentration in theregion around the first end portion 5 a and the second end portion 5 bof the outer peripheral wall 5 e can be alleviated.

As described above, the outer radial side apex portion 5 c of the holeportion 5 of the first hole portion group 6 is located at theintersection point X between the virtual line L4 and the virtual line L6or is located radially outward of the intersection point X. Thus, sinceit is possible to reduce the bending stress generated in the first outerperipheral wall 5 f and the second outer peripheral wall 5 g when thecentrifugal force is generated, stress concentration in the regionaround the first end portion 5 a and the second end portion 5 b of theouter peripheral wall 5 e by the centrifugal force can be alleviated.

Since, in the hole portion 5 of the first hole portion group 6, theinner peripheral wall 5 h has a substantially linear shape, a forceacting on the inner peripheral wall 5 h when the centrifugal force actson the outer radial side apex portion 5 c of the hole portion 5 hasalmost no radial component in the circumferential central portion of theinner peripheral wall 5 h. Therefore, it is possible to suppress thewidening of the rotor shaft hole 4 due to the centrifugal force and tosuppress the reduction of the interference due to the widening of therotor shaft hole 4.

As illustrated in FIG. 4A, each hole portion 8 of the second holeportion group 9 has a substantially rectangular shape convex on bothsides in the circumferential direction and both sides in the radialdirection. Each hole portion 8 of the second hole portion group 9 has afirst end portion 8 a and a second end portion 8 b forming bothcircumferential end portions, an outer radial side apex portion 8 cwhich has a radial distance from the center CL of the rotor core 1longer than that of the first end portion 8 a and the second end portion8 b and forms a radially outer apex portion, and an inner radial sideapex portion 8 d which has a radial distance from the center CL of therotor core 1 shorter than that of the first end portion 8 a and thesecond end portion 8 b and forms a radially inner apex portion.Therefore, the hole area of the hole portion 8 can be increased, andthus the weight reduction of the rotor core 1 can be achieved. Inaddition, stress concentration in the first end portion 8 a and thesecond end portion 8 b due to the centrifugal force and the tighteningload of the rotor shaft 2 can be alleviated.

Further, the hole portion 8 of the second hole portion group 9 includesan outer peripheral wall 8 e which has a first outer peripheral wall 8 fextending substantially linearly from the first end portion 8 a to theouter radial side apex portion 8 c and a second outer peripheral wall 8g extending substantially linearly from the second end portion 8 b tothe outer radial side apex portion 8 c. In addition, the hole portion 8of the second hole portion group 9 includes an inner peripheral wall 8 hwhich has a first inner peripheral wall 8 i extending substantiallylinearly from the first end portion 8 a to the inner radial side apexportion 8 d and a second inner peripheral wall 8 j extendingsubstantially linearly from the second end portion 8 b to the innerradial side apex portion 8 d.

Each hole portion 8 of the second hole portion group 9 is deformed sothat the outer radial side apex portion 8 c is pulled radially outwardwith respect to the centrifugal force. By the deformation of the holeportion 8, the centrifugal force is absorbed in the hole portion 8.Therefore, since it is possible to suppress the centrifugal force frombeing transmitted to the radially inner side of the rotor core 1, it ispossible to suppress the widening of the rotor shaft hole 4 due to thecentrifugal force and to suppress the reduction of the interference dueto the widening of the rotor shaft hole 4.

Furthermore, the hole portion 8 of the second hole portion group 9 isdeformed so that the inner radial side apex portion 8 d is pushedradially outward with respect to the tightening load of the rotor shaft2. By the deformation of the hole portion 8, the tightening load of therotor shaft 2 is absorbed in the hole portion 8. Therefore, since it ispossible to suppress the tightening load of the rotor shaft 2 from beingtransmitted to the outside in the radial direction of the rotor core 1,it is possible to suppress the deformation of the outer peripheralportion of the rotor core 1 due to the tightening load of the rotorshaft 2.

In the hole portion 8 of the second hole portion group 9, the outerradial side apex portion 8 c and the inner radial side apex portion 8 dare located on the virtual line L1 and has a symmetrical shape withrespect to the virtual line L1.

The outer radial side apex portion 8 c of the hole portion 8 of thesecond hole portion group 9 is located at an intersection point Ybetween a virtual line L8 which is orthogonal to a virtual line L7connecting the center CL of the rotor core 1 and the first end portion 8a and passes through the first end portion 8 a and a virtual line L10which is orthogonal to a virtual line L9 connecting the center CL of therotor core 1 and the second end portion 8 b and passes through thesecond end portion 8 b, or is located radially outward of theintersection point Y. In the embodiment, the outer radial side apexportion 8 c of the hole portion 8 of the second hole portion group 9 islocated radially outward of the intersection point Y.

Therefore, since the hole portion 8 of the second hole portion group 9can reduce the bending stress generated in the first outer peripheralwall 8 f and the second outer peripheral wall 8 g when the centrifugalforce is generated, it is possible to alleviate the stress concentrationin the region around the first end portion 8 a and the second endportion 8 b of the outer peripheral wall 8 e due to the centrifugalforce.

An angle θ1 formed by the first outer peripheral wall 8 f and the secondouter peripheral wall 8 g of the hole portion 8 of the second holeportion group 9 and an angle θ2 formed by the first inner peripheralwall 8 i and the second inner peripheral wall 8 j of the hole portion 8of the second hole portion group 9 satisfy the following equation (1),taking an angle formed by the first end portion 8 a and the second endportion 8 b at the center CL of the rotor core 1 as ϕ. Each of θ1 and θ2is an angle larger than 0° and smaller than 180°. ϕ is an angle largerthan 0° and smaller than 360°/(the number of magnet pole portions 20 ofrotor core 1). In the embodiment, since the number of magnet poleportions 20 of the rotor core 1 is 12, ϕ is an angle larger than 0° andsmaller than 30°.θ1+2ϕ≥θ2≥θ1   (1)

As illustrated in FIG. 4B, when a centrifugal force F1 acts on the outerradial side apex portion 8 c of the hole portion 8, a force Fa acting ina direction toward the intersection point Y along the virtual line L8 isgenerated in the first end portion 8 a and a force Fb acting in adirection toward the intersection point Y along the virtual line L10 isgenerated in the second end portion 8 b, and further, a force F2 actingradially inward along the imaginary line L1 is generated in the innerradial side apex portion 8 d.

Since the hole portion 8 of the second hole portion group 9 has asymmetrical shape with respect to the virtual line L1, assuming that thetension generated in the first outer peripheral wall 8 f by thecentrifugal force F1 is f1, the following equation (2) is established.F1=2·f1 cos (θ1/2)   (2)

Similarly, assuming that the tension generated in the first innerperipheral wall 8 i by the force F2 is f2, the following equation (3) isestablished.F2=2·f2·cos (θ2/2)   (3)

Assuming that an angle formed by the first outer peripheral wall 8 f andthe virtual line L8 at the first end portion 8 a of the hole portion 8of the second hole portion group 9 is θ3 and an angle formed by thefirst inner peripheral wall 8 i and the virtual line L8 at the first endportion 8 a is θ4, the following equation (4) is established.Fa=f1·cos θ3+f2·cos θ4   (4)

Further, since the force Fa acting on the first end portion 8 a actsonly in a direction toward the intersection point Y along the virtualline L8, the components orthogonal to the virtual line L8 cancel eachother and the following equation (5) is established.f1·sin θ3=f2·sin θ4   (5)

Further, since the hole portion 8 of the second hole portion group 9 hasa symmetrical shape with respect to the virtual line L1, the followingequations (6) and (7) are established.θ3=90°−(θ1+ϕ)/2   (6)θ4=90°−(θ2−ϕ)/2   (7)

From the equations (2) to (7), the following equation (8) is derived byeliminating f1, f2, θ3, θ4, and Fa.

$\begin{matrix}{{F\; 2} = {F\;{1 \cdot \frac{\cos\frac{{\theta\; 1} + \phi}{2}}{\cos\frac{{\theta\; 2} - \phi}{2}} \cdot \frac{\cos\frac{\theta\; 2}{2}}{\cos\frac{\theta\; 1}{2}}}}} & (8)\end{matrix}$

Here, since θ1, θ2, and ϕ satisfy the equation (1), the followingequations (9) and (10) are established.θ1+ϕ≥θ2−ϕ  (9)θ2≥θ1   (10)

Therefore, the following equations (11) and (12) are established.

$\begin{matrix}{\frac{\cos\frac{{\theta\; 1} + \phi}{2}}{\cos\frac{{\theta\; 2} - \phi}{2}} \leqq 1} & (11) \\{\frac{\cos\frac{\theta\; 2}{2}}{\cos\frac{\theta\; 1}{2}} \leqq 1} & (12)\end{matrix}$

Therefore, from the equations (8), (11), and (12), the centrifugal forceF1 acting on the outer radial side apex portion 8 c of the hole portion8 and the force F2 acting on the inner radial side apex portion 8 d bythe centrifugal force F1 always satisfy F2≤F1.

That is, in the hole portion 8 of the second hole portion group 9, theforce F2 acting on the inner radial side apex portion 8 d by thecentrifugal force F1 is always smaller than the centrifugal force F1acting on the outer radial side apex portion 8 c. As a result, since thereaction force of the force F2 acting on the inner radial side apexportion 8 d is always smaller than the centrifugal force F1, it ispossible to suppress the widening of the rotor shaft hole 4 due to thecentrifugal force and to suppress the reduction of the interference dueto the widening of the rotor shaft hole 4.

Further, in the hole portion 8 of the second hole portion group 9, theouter radial side apex portion 8 c and the inner radial side apexportion 8 d are located on the virtual line L1 and the hole portion 8has a symmetrical shape with respect to the virtual line L1. Therefore,the hole portion 8 can suppress the widening of the rotor shaft 4 due tothe centrifugal force and suppress the reduction of the interference dueto the widening of the rotor shaft 4, and it is possible to moreeffectively absorb the tightening load of the rotor shaft 2.

As illustrated in FIG. 5, the hole portion 11 of the third hole portiongroup 12 also has the same shape as the hole portion 8 of the secondhole portion group 9.

Each hole portion 11 of the third hole portion group 12 has asubstantially rectangular shape convex on both sides in thecircumferential direction and both sides in the radial direction. Eachhole portion 11 of the third hole portion group 12 has a first endportion 11 a and a second end portion 11 b forming both circumferentialend portions, an outer radial side apex portion 11 c which has a radialdistance from the center CL of the rotor core 1 longer than that of thefirst end portion 11 a and the second end portion 11 b and forms aradially outer apex portion, and an inner radial side apex portion 11 dwhich has a radial distance from the center CL of the rotor core 1shorter than that of the first end portion 11 a and the second endportion 11 b and forms a radially inner apex portion. Therefore, thehole area of the hole portion 11 can be increased, and thus the weightreduction of the rotor core 1 can be achieved. In addition, stressconcentration in the first end portion 11 a and the second end portion11 b due to the centrifugal force and the tightening load of the rotorshaft 2 can be alleviated.

Further, the hole portion 11 of the third hole portion group 12 includesan outer peripheral wall 11 e which has a first outer peripheral wall 11f extending substantially linearly from the first end portion 11 a tothe outer radial side apex portion 11 c and a second outer peripheralwall 11 g extending substantially linearly from the second end portion11 b to the outer radial side apex portion 11 c. In addition, the holeportion 11 of the third hole portion group 12 includes an innerperipheral wall 11 h which has a first inner peripheral wall 11 iextending substantially linearly from the first end portion 11 a to theinner radial side apex portion 11 d and a second inner peripheral wall11 j extending substantially linearly from the second end portion 11 bto the inner radial side apex portion 11 d.

Each hole portion 11 of the third hole portion group 12 is deformed sothat the outer radial side apex portion 11 c is pulled radially outwardwith respect to the centrifugal force. By the deformation of the holeportion 11, the centrifugal force is absorbed in the hole portion 11.

Therefore, since it is possible to suppress the centrifugal force frombeing transmitted to the radially inner side of the rotor core 1, it ispossible to suppress the widening of the rotor shaft hole 4 due to thecentrifugal force and to suppress the reduction of the interference dueto the widening of the rotor shaft hole 4.

Furthermore, the hole portion 11 of the third hole portion group 12 isdeformed so that the inner radial side apex portion 11 d is pushedradially outward with respect to the tightening load of the rotor shaft2. By the deformation of the hole portion 11, the tightening load of therotor shaft 2 is absorbed in the hole portion 11. Therefore, since it ispossible to suppress the tightening load of the rotor shaft 2 from beingtransmitted to the outside in the radial direction of the rotor core 1,it is possible to suppress the deformation of the outer peripheralportion of the rotor core 1 due to the tightening load of the rotorshaft 2.

In the hole portion 11 of the second hole portion group 12, the outerradial side apex portion 11 c and the inner radial side apex portion 11d are located on the virtual line L2 and has a symmetrical shape withrespect to the virtual line L2.

The outer radial side apex portion 11 c of the hole portion 11 of thethird hole portion group 12 is located at an intersection point Zbetween a virtual line L12 which is orthogonal to a virtual line L11connecting the center CL of the rotor core 1 and the first end portion11 a and passes through the first end portion 11 a and a virtual lineL14 which is orthogonal to a virtual line L13 connecting the center CLof the rotor core 1 and the second end portion 11 b and passes throughthe second end portion 11 b, or is located radially outward of theintersection point Z. In the embodiment, the outer radial side apexportion 11 c of the hole portion 11 of the third hole portion group 12is located radially outward of the intersection point Z.

Therefore, since the hole portion 11 of the third hole portion group 12can reduce the bending stress generated in the first outer peripheralwall 11 f and the second outer peripheral wall 11 g when the centrifugalforce is generated, it is possible to alleviate the stress concentrationin the region around the first end portion 11 a and the second endportion 11 b of the outer peripheral wall 11 e by the centrifugal force.

An angle θ5 formed by the first outer peripheral wall 11 f and thesecond outer peripheral wall 11 g of the hole portion 11 of the thirdhole portion group 12 and an angle θ6 formed by the first innerperipheral wall 11 i and the second inner peripheral wall 11 j of thehole portion 11 of the third hole portion group 12 satisfy the followingequation (13), taking an angle formed by the first end portion 11 a andthe second end portion 11 b at the center CL of the rotor core 1 as σ.Each of θ5 and θ6 is an angle larger than 0° and smaller than 180°. σ isan angle larger than 0° and smaller than 360°/(the number of magnet poleportions 20 of rotor core 1). In the embodiment, since the number ofmagnet pole portions 20 of the rotor core 1 is 12, σ is an angle largerthan 0° and smaller than 30°.θ5+2σ≥θ6≥θ5   (13)

Therefore, in the hole portion 11 of the third hole portion group 12,the force acting on the inner radial side apex portion 11 d by thecentrifugal force is always smaller than the centrifugal force acting onthe outer radial side apex portion 11 c. As a result, it is possible tosuppress the widening of the rotor shaft hole 4 due to the centrifugalforce and to suppress the reduction of the interference due to thewidening of the rotor shaft hole 4.

Further, in the hole portion 11 of the third hole portion group 12, theouter radial side apex portion 11 c and the inner radial side apexportion 11 d are located on the virtual line L2 and the hole portion 11has a symmetrical shape with respect to the virtual line L2. Therefore,the hole portion 11 can suppress the widening of the rotor shaft 4 dueto the centrifugal force and suppress the reduction of the interferencedue to the widening of the rotor shaft 4, and it is possible to moreeffectively absorb the tightening load of the rotor shaft 2.

Also, the plurality of hole portions 5 of the first hole portion group 6have all the same shape, and the plurality of hole portions 8 of thesecond hole portion group 9 have all the same shape, and further theplurality of hole portions 11 of the third hole portion group 12 haveall the same shape. Furthermore, the outer radial side apex portions 5 cof the plurality of hole portions 5 of the first hole portion group 6are arranged such that all radial distances from the center CL of therotor core 1 are equal. The outer radial side apex portions 8 c of theplurality of hole portions 8 of the second hole portion group 9 arearranged such that all radial distances from the center CL of the rotorcore 1 are equal and the inner radial side apex portions 8 d of theplurality of hole portions 8 of the second hole portion group 9 arearranged such that all radial distances from the center CL of the rotorcore 1 are equal. The outer radial side apex portions 11 c of theplurality of hole portions 11 of the third hole portion group 12 arearranged such that all radial distances from the center CL of the rotorcore 1 are equal and the inner radial side apex portions 11 d of theplurality of hole portions 11 of the third hole portion group 12 arearranged such that all radial distances from the center CL of the rotorcore 1 are equal.

In this way, the first hole portion group 6, the second hole portiongroup 9, and the third hole portion group 12 can receive the centrifugalforce in a well-balanced manner and it is possible to equalize thedeformation in the plurality of hole portions 5 of the first holeportion group 6, the deformation in the plurality of hole portions 8 ofthe second hole portion group 9, and the deformation in the plurality ofhole portions 11 of the third hole portion group 12.

The first end portions 8 a and 11 a, the second end portions 8 b and 11b, the outer radial side apex portions 8 c and 11 c, and the innerradial side apex portions 8 d and 11 d of the hole portions 8 and 11 ofthe embodiment all have rounded corners in which the corners arerounded. However, the shapes of first end portion 8 a and 11 a, thesecond end portion 8 b and 11 b, the outer radial side apex portions 8 cand 11 c, and the inner radial side apex portions 8 d and 11 d can bechanged as appropriate.

As illustrated in FIG. 6, the hole portion 8 of the second hole portiongroup 9 and the hole portion 11 of the third hole portion group 12 mayhave a substantially triangular shape which is convex outward in theradial direction. In this case, the inner peripheral walls 8 h and 11 hof the hole portion 8 of the second hole portion group 9 and the holeportion 11 of the third hole portion group 12 are substantiallyorthogonal to the virtual line L1 and the virtual line L2, respectively,and have a substantially straight line shape extending from the firstend portions 8 a and 11 a to the second end portions 8 b and 11 b,respectively. In this way, the forces acting on the inner peripheralwalls 8 h and 11 h when the centrifugal forces act on the outer radialside apex portions 8 c and 11 c of the hole portion 8 and the holeportion 11 have almost no radial components in the circumferentialcenter portions of the inner peripheral walls 8 h and 11 h. For thisreason, it is possible to suppress the widening of the rotor shaft hole4 due to the centrifugal force and to suppress the reduction of theinterference due to the widening of the rotor shaft hole 4.

Second Embodiment

Next, a rotor core 1A according to a second embodiment of the inventionwill be described with reference to FIG. 7. In the followingdescription, the same constituent components as those of the rotor core1 of the first embodiment are denoted by the same reference numerals andthe descriptions thereof will be omitted or simplified. In the rotorcore 1 of the first embodiment, one magnet pole portion 20 isconstituted by three magnets 3 arranged in three magnet insertion holes14 arranged in an arc shape. However, in the rotor core 1A of the secondembodiment, one magnet pole portion 20 is constituted by two magnets 3arranged in two magnet insertion holes 14 arranged in a V shape.Moreover, although the rotor core 1 of the first embodiment forms twoannular portions 10 and 13 by three hole portion groups 6, 9, and 12, inthe rotor core 1A of the second embodiment, one annular portion 10 isformed by two hole portion groups 6 and 9. The shapes and arrangementsof the plurality of hole portions 5 of the first hole portion group 6and the plurality of hole portions 8 of the second hole portion group 9are the same as in the first embodiment.

Therefore, each hole portion 8 of the second hole portion group 9 isdeformed so that the outer radial side apex portion 8 c is pulledradially outward with respect to the centrifugal force. By thedeformation of the hole portion 8, the centrifugal force is absorbed inthe hole portion 8. Therefore, since it is possible to suppress thecentrifugal force from being transmitted to the radially inner side ofthe rotor core 1, it is possible to suppress the widening of the rotorshaft hole due to the centrifugal force and to suppress the reduction ofthe interference due to the widening of the rotor shaft hole.

Furthermore, the hole portion 8 of the second hole portion group 9 isdeformed so that the inner radial side apex portion 8 d is pushedradially outward with respect to the tightening load of the rotor shaft2. By the deformation of the hole portion 8, the tightening load of therotor shaft 2 is absorbed by the hole portion 8. Therefore, since it ispossible to suppress the tightening load of the rotor shaft 2 from beingtransmitted to the radially outer side of the rotor core 1, it ispossible to suppress the deformation of the outer peripheral portion ofthe rotor core 1 due to the tightening load of the rotor shaft 2.

Further, in the hole portion 8 of the second hole portion group 9, theforce acting on the inner radial side apex portion 8 d by thecentrifugal force is always smaller than the centrifugal force acting onthe outer radial side apex portion 8 c. As a result, since the reactionforce of the force acting on the inner radial side apex portion 8 d isalways smaller than the centrifugal force, it is possible to suppressthe widening of the rotor shaft hole 4 due to the centrifugal force andto suppress the reduction of the interference due to the widening of therotor shaft hole 4.

In the embodiment described above, modifications, improvements, and thelike can be made as appropriate.

At least the following matters are described in the presentspecification. In addition, although the constituent componentscorresponding to those in the embodiment described above are describedin the parenthesis, it is not limited thereto.

(1) A rotor core (rotor core 1) including

a rotor shaft hole (rotor shaft hole 4) into which a rotor shaft (rotorshaft 2) is tightened,

a first hole portion group (first hole portion group 6) provided on anouter side of the rotor shaft hole in a radial direction and having aplurality of hole portions (hole portions 5) arranged in acircumferential direction,

a shaft holding portion (shaft holding portion 7) provided between therotor shaft hole and the first hole portion group in the radialdirection,

a second hole portion group (second hole portion group 9) provided on anouter side of the first hole portion group in the radial direction andhaving a plurality of hole portions (hole portions 8) arranged in thecircumferential direction,

a first annular portion (first annular portion 10) provided between thefirst hole portion group and the second hole portion group in the radialdirection, and

an electromagnetic portion (electromagnetic portion 15) provided on anouter side of the second hole portion group in the radial direction andhaving a plurality of magnet insertion holes (magnet insertion holes 14)into which magnets (magnets 3) are respectively inserted, wherein

each hole portion of the second hole portion group is arranged tointersect with a virtual extension line (virtual line L1) of a rib (rib16) formed between the adjacent hole portions of the first hole portiongroup;

each hole portion of the second hole portion group includes

a first end portion (first end portion 8 a) and a second end portion(second end portion 8 b) forming both circumferential end portions,

an outer radial side apex portion (outer radial side apex portion 8 c)having a radial distance from a center (center CL) of the rotor corelonger than that of the first end portion and the second end portion andforming a radially outer apex portion, and

an outer peripheral wall (outer peripheral wall 8 e) having a firstouter peripheral wall (first outer peripheral wall 8 f) extending fromthe first end portion to the outer radial side apex portion and a secondouter peripheral wall (second outer peripheral wall 8 g) extending fromthe second end portion to the outer radial side apex portion; and

the outer radial side apex portion is located at an intersection point(intersection point Y) between a virtual line (virtual line L8) which isorthogonal to a virtual line (virtual line L7) connecting the center ofthe rotor core and the first end portion and passes through the firstend portion and a virtual line (virtual line L10) which is orthogonal toa virtual line (virtual line L9) connecting the center of the rotor coreand the second end portion and passes through the second end portion, oris located radially outward of the intersection point.

According to (1), each hole portion of the second hole portion group isarranged to intersect with the virtual line of the rib formed betweenthe adjacent hole portions of the first hole portion group. That is, thehole portions of the first hole portion group and the hole portions ofthe second hole portion group are alternately arranged in thecircumferential direction. Thus, the centrifugal force transmitted tothe first annular portion through the rib located between the adjacenthole portions of the second hole portion group can be absorbed by thehole portions of the first hole portion group.

Further, each hole portion of the second hole portion group can reducethe bending stress generated in the first outer peripheral wall and thesecond outer peripheral wall when the centrifugal force is generated,stress concentration in the region around the first end portion and thesecond end portion of the outer peripheral wall by the centrifugal forcecan be alleviated.

(2) The rotor core according to (1), wherein

respective hole portions of the second hole portion group are arrangedsuch that radial distances from the center of the rotor core of theouter radial side apex portions are equal.

According to (2), since respective hole portions of the second holeportion group are arranged such that radial distances from the center ofthe rotor core of the outer radial side apex portions are equal, thecentrifugal force can be received by the second hole portion group in abalanced manner.

(3) The rotor core according to (1) or (2), where

respective hole portions of the second hole portion group have the sameshape.

According to (3), since respective hole portions of the second holeportion group have the same shape, the centrifugal force can be receivedby the second hole portion group in a balanced manner.

(4) The rotor core according to any one of (1) to (3), wherein

each hole portion of the second hole portion group further includes

an inner radial side apex portion (inner radial side apex portion 8 d)having a radial distance from the center of the rotor core shorter thanthat of the first end portion and the second end portion and forming aradially inner apex portion, and

an inner peripheral wall (inner peripheral wall 8 h) having a firstinner peripheral wall (first inner peripheral wall 8 i) extending fromthe first end portion to the inner radial side apex portion and a secondinner peripheral wall (second inner peripheral wall 8 j) extending fromthe second end portion to the inner radial side apex portion and

has a substantially rectangular shape; and

when an angle formed by the first outer peripheral wall and the secondouter peripheral wall of the hole portion of the second hole portiongroup is set as θ1, and an angle formed by the first inner peripheralwall and the second inner peripheral wall is set as θ2, and further anangle formed by the first end portion and the second end portion at thecenter of the rotor core is set as ϕ, θ1+2ϕ≥θ2≥θ1 is satisfied.

According to (4), since each hole portion of the second hole portiongroup satisfies θ1+2ϕ≥θ2≥θ1, in each hole portion of the second holeportion group, the force acting on the inner radial side apex portion bythe centrifugal force is always smaller than the centrifugal forceacting on the outer radial side apex portion. Therefore, it is possibleto suppress the widening of the rotor shaft hole due to the centrifugalforce and to suppress the reduction of the interference due to thewidening of the rotor shaft hole.

(5) The rotor core according to any one of (1) to (4), wherein

each hole portion of the first hole portion group includes

a first end portion (first end portion 5 a) and a second end portion(second end portion 5 b) forming both circumferential end portions,

an outer radial side apex portion (outer radial side apex portion 5 c)having a radial distance from the center of the rotor core than longerthat of the first end portion and the second end portion and forming aradially outer apex portion, and

an outer peripheral wall (outer peripheral wall 5 e) having a firstouter peripheral wall (first outer peripheral wall 5 f) extending fromthe first end portion to the outer radial side apex portion and a secondouter peripheral wall (second outer peripheral wall 5 g) extending fromthe second end portion to the outer radial side apex portion; and

the outer radial side apex portion is located at an intersection point(intersection point X) between a virtual line (virtual line L4) which isorthogonal to a virtual line (virtual line L3) connecting the center ofthe rotor core and the first end portion and passes through the firstend portion and a virtual line (virtual line L6) which is orthogonal toa virtual line (virtual line L5) connecting the center of the rotor coreand the second end portion and passes through the second end portion oris located radially outward of the intersection point.

According to (5), since each hole portion of the first hole portiongroup can reduce the bending stress generated in the first outerperipheral wall and the second outer peripheral wall when thecentrifugal force is generated, stress concentration in the regionaround the first end portion and the second end portion of the outerperipheral wall by the centrifugal force can be alleviated.

(6) The rotor core according to any one of (1) to (5), wherein

the rotor core further includes

a third hole portion group (third hole portion group 12) provided on anouter side of the second hole portion group and an inner side of theelectromagnetic portion in the radial direction and having a pluralityof hole portions (hole portions 11) arranged in the circumferentialdirection, and

a second annular portion (second annular portion 13) provided betweenthe second hole portion group and the third hole portion group in theradial direction;

each hole portion of the third hole portion group is arranged tointersect with a virtual extension line (virtual line L2) of a rib (rib17) formed between the adjacent hole portions of the second hole portiongroup;

each hole portion of the third hole portion group includes

a first end portion (first end portion 11 a) and a second end portion(second end portion 11 b) forming both circumferential end portions,

an outer radial side apex portion (outer radial side apex portion 11 c)having a radial distance from the center of the rotor core longer thanthat of the first end portion and the second end portion and forming aradially outer apex portion, and

an outer peripheral wall (outer peripheral wall 11 e) having a firstouter peripheral wall (first outer peripheral wall 11 f) extending fromthe first end portion to the outer radial side apex portion and a secondouter peripheral wall (second outer peripheral wall 11 g) extending fromthe second end portion to the outer radial side apex portion; and

the outer radial side apex portion is located at an intersection point(intersection point Z) between a virtual line (virtual line L12) whichis orthogonal to a virtual line (virtual line L11) connecting the centerof the rotor core and the first end portion and passes through the firstend portion and a virtual line (virtual line L14) which is orthogonal toa virtual line (virtual line L13) connecting the center of the rotorcore and the second end portion and passes through the second endportion, or is located radially outward of the intersection point.

According to (6), each hole portion of the third hole portion group isarranged to intersect with the virtual line of the rib formed betweenthe adjacent hole portions of the second hole portion group. That is,the hole portions of the second hole portion group and the hole portionsof the third hole portion group are alternately arranged in thecircumferential direction. Thus, the centrifugal force transmitted tothe second annular portion through the rib located between the adjacenthole portions of the third hole portion group can be absorbed by thehole portions of the second hole portion group.

Further, since each hole portion of the third hole portion group canreduce the bending stress generated in the first outer peripheral walland the second outer peripheral wall when the centrifugal force isgenerated, stress concentration in the region around the first endportion and the second end portion of the outer peripheral wall by thecentrifugal force can be alleviated.

(7) The rotor core according to (6), wherein

respective hole portions of the third hole portion group are arrangedsuch that radial distances from the center of the rotor core of theouter radial side apex portions are equal.

According to (7), since respective hole portions of the third holeportion group are arranged such that radial distances from the center ofthe rotor core of the outer radial side apex portions are equal, thecentrifugal force can be received by the third hole portion group in abalanced manner.

(8) The rotor core according to (6) or (7), wherein

respective hole portions of the third hole portion group have the sameshape.

According to (8), since respective hole portions of the third holeportion group have the same shape, the centrifugal force can be receivedby the third hole portion group in a balanced manner.

(9) The rotor core according to any one of (6) to (8), wherein

each hole portion of the third hole portion group further includes,

an inner radial side apex portion (inner radial side apex portion 11 d)having a radial distance from the center of the rotor core shorter thanthat of the first end portion and the second end portion and forming aradially inner apex portion, and

an inner peripheral wall (inner peripheral wall 11 h) having a firstinner peripheral wall (first inner peripheral wall 11 i) extending fromthe first end portion to the inner radial side apex portion and a secondinner peripheral wall (second inner peripheral wall 11 j) extending fromthe second end portion to the inner radial side apex portion and

has a substantially rectangular shape; and

when an angle formed by the first outer peripheral wall and the secondouter peripheral wall of the hole portion of the third hole portiongroup is set as θ5, and an angle formed by the first inner peripheralwall and the second inner peripheral wall is set as θ6, and further anangle formed by the first end portion and the second end portion at thecenter of the rotor core is set as σ, θ5+2σ≥θ6≥θ5 is satisfied.

According to (9), since each hole portion of the third hole portiongroup satisfies θ5+2σ≥θ6≥θ5, in each hole portion of the third holeportion group, the force acting on the inner radial side apex portion bythe centrifugal force is always smaller than the centrifugal forceacting on the outer radial side apex portion. Therefore, it is possibleto suppress the widening of the rotor shaft hole due to the centrifugalforce and to suppress the reduction of the interference due to thewidening of the rotor shaft hole.

The invention claimed is:
 1. A rotor core comprising: a rotor shaft holeinto which a rotor shaft is tightened; a first hole portion groupprovided on an outer side of the rotor shaft hole in a radial directionand having a plurality of hole portions arranged in a circumferentialdirection; a shaft holding portion provided between the rotor shaft holeand the first hole portion group in the radial direction; a second holeportion group provided on an outer side of the first hole portion groupin the radial direction and having a plurality of hole portions arrangedin the circumferential direction; a first annular portion providedbetween the first hole portion group and the second hole portion groupin the radial direction; and an electromagnetic portion provided on anouter side of the second hole portion group in the radial direction andhaving a plurality of magnet insertion holes into which magnets arerespectively inserted, wherein: each hole portion of the second holeportion group is arranged to intersect with a virtual extension line ofa rib formed between the adjacent hole portions of the first holeportion group, each hole portion of the second hole portion groupincludes: a first end portion and a second end portion forming bothcircumferential end portions; an outer radial side apex portion having aradial distance from a center of the rotor core longer than that of thefirst end portion and the second end portion and forming a radiallyouter apex portion; and an outer peripheral wall having a first outerperipheral wall extending from the first end portion to the outer radialside apex portion and a second outer peripheral wall extending from thesecond end portion to the outer radial side apex portion, the outerradial side apex portion is located at an intersection point between avirtual line which is orthogonal to a virtual line connecting the centerof the rotor core and the first end portion and passes through the firstend portion and a virtual line which is orthogonal to a virtual lineconnecting the center of the rotor core and the second end portion andpasses through the second end portion or is located radially outward ofthe intersection point, a radially inner end portion of each holeportion of the second hole portion group is positioned radially outwardof a radially outer end portion of each hole portion of the first holeportion group, each hole portion of the second hole portion groupfurther includes: an inner radial side apex portion having a radialdistance from the center of the rotor core shorter than that of thefirst end portion and the second end portion and forming a radiallyinner apex portion; and an inner peripheral wall having a first innerperipheral wall extending from the first end portion to the inner radialside apex portion and a second inner peripheral wall extending from thesecond end portion to the inner radial side apex portion, each holeportion of the second hole portion group has a substantially rectangularshape, and when an angle formed by the first outer peripheral wall andthe second outer peripheral wall of the hole portion of the second holeportion group is set as θ1, and an angle formed by the first innerperipheral wall and the second inner peripheral wall is set as θ2, andfurther an angle formed by the first end portion and the second endportion at the center of the rotor core is set as ϕ, θ1+2ϕ≥θ2≥θ1 issatisfied.
 2. The rotor core according to claim 1, wherein respectivehole portions of the second hole portion group are arranged such thatradial distances from the center of the rotor core of the outer radialside apex portions are equal.
 3. The rotor core according to claim 1,wherein respective hole portions of the second hole portion group havethe same shape.
 4. The rotor core according to claim 1, wherein: eachhole portion of the first hole portion group includes: a first endportion and a second end portion forming both circumferential endportions; an outer radial side apex portion having a radial distancefrom the center of the rotor core longer than that of the first endportion and the second end portion and forming a radially outer apexportion; and an outer peripheral wall having a first outer peripheralwall extending from the first end portion to the outer radial side apexportion and a second outer peripheral wall extending from the second endportion to the outer radial side apex portion; and the outer radial sideapex portion is located at an intersection point between a virtual linewhich is orthogonal to a virtual line connecting the center of the rotorcore and the first end portion and passes through the first end portionand a virtual line which is orthogonal to a virtual line connecting thecenter of the rotor core and the second end portion and passes throughthe second end portion or is located radially outward of theintersection point.
 5. The rotor core according to claim 1, wherein: therotor core further includes: a third hole portion group provided on anouter side of the second hole portion group and an inner side of theelectromagnetic portion in the radial direction and having a pluralityof hole portions arranged in the circumferential direction; and a secondannular portion provided between the second hole portion group and thethird hole portion group in the radial direction; each hole portion ofthe third hole portion group is arranged to intersect with a virtualextension line of a rib formed between the adjacent hole portions of thesecond hole portion group; each hole portion of the third hole portiongroup includes: a first end portion and a second end portion formingboth circumferential end portions; an outer radial side apex portionhaving a radial distance from the center of the rotor core longer thanthat of the first end portion and the second end portion and forming aradially outer apex portion; and an outer peripheral wall having a firstouter peripheral wall extending from the first end portion to the outerradial side apex portion and a second outer peripheral wall extendingfrom the second end portion to the outer radial side apex portion; andthe outer radial side apex portion is located at an intersection pointbetween a virtual line which is orthogonal to a virtual line connectingthe center of the rotor core and the first end portion and passesthrough the first end portion and a virtual line which is orthogonal toa virtual line connecting the center of the rotor core and the secondend portion and passes through the second end portion or is locatedradially outward of the intersection point.
 6. The rotor core accordingto claim 5, wherein respective hole portions of the third hole portiongroup are arranged such that radial distances from the center of therotor core of the outer radial side apex portions are equal.
 7. Therotor core according to claim 5, wherein respective hole portions of thethird hole portion group have the same shape.
 8. The rotor coreaccording to claim 5, wherein: each hole portion of the third holeportion group further includes: an inner radial side apex portion havinga radial distance from the center of the rotor core shorter than that ofthe first end portion and the second end portion and forming a radiallyinner apex portion; and an inner peripheral wall having a first innerperipheral wall extending from the first end portion to the inner radialside apex portion and a second inner peripheral wall extending from thesecond end portion to the inner radial side apex portion and has asubstantially rectangular shape; and when an angle formed by the firstouter peripheral wall and the second outer peripheral wall of the holeportion of the third hole portion group is set as θ5, and an angleformed by the first inner peripheral wall and the second innerperipheral wall is set as θ6, and further an angle formed by the firstend portion and the second end portion at the center of the rotor coreis set as σ, θ5+2σ≥θ6≥θ5 is satisfied.